"maintain" is more like keeping it in its original condition, or taking care of it well while holding it.“maintain”更偏向于保持它原来的状态，或者在拥有它的时候好好照顾它。
"Maintence" is work done to make sure that something will still function properly(repairing a car, changing a broken piece on a computer).“维护”是指确保某些东西仍然正常工作的工作（修理汽车、更换电脑上的碎片）。
*"You have to maintain a healthy weight."“你必须保持健康的体重。”*"If you want to maintain a good relationship.Make sure to not argue about small things"“如果你想保持良好的关系，一定不要为小事争吵”*"I maintain my car every month"“我每个月都保养我的车”
"sustain" means to provide enough of what sb/sth needs in order to live or exist，“维持”是指为某人/某物的生存提供足够的所需
or to make sth continue for some time without becoming less.或使某事持续一段时间而不减少。
*"This plant is sustained by water and sunlight." “这种植物靠水和阳光维持生长。”*"He can sustain a long conversation." “他能坚持很长时间的谈话。”*"Oil money is the only thing sustaining our economy.“石油货币是维持我们经济的唯一东西*"This food is enough to sustain us for a while"“这些食物足够我们维持一段时间”
"keep" is to hold something or make something stay with you “保持”是指拥有某物或使某物留在你身边
or to stay in a particular condition or position.或保持在某一特定条件或位置。
*We huddled together to keep warm.我们挤在一起来保暖。*She had trouble keeping her balance .她保持平衡有困难。
现在你分清楚了吗？